feat(web): theme import/export, more themable colors, hideable local/ical calendars

Theme:
- Export/import themes via UI as <date>_<time>.theme (JSON, readable keys,
  link to THEME.md). Import is partial-aware: only present params are written
  (server accepts partial); prompts before importing files with unknown params.
- Dynamic favicon + theme-color tinted to the primary colour on load/save.
- New themable colours: general hover-highlight, day hover/selected/bg,
  today background, plus two unified sidebar action-icon colours
  (inactive/active) covering bell, hide, delete and read-only icons.
- All new colours are per-setting syncable; documented in THEME.md.

UX:
- Styled confirm dialog (#modal-confirm) replaces window.confirm() for
  calendar delete and account disconnect.
- Birthday/local calendars and iCal subscriptions can now be hidden from the
  sidebar via Settings (new sidebar_hidden column + hide toggle).

Backend: additive nullable columns + idempotent migrations for user_settings
colours and local_calendars/ical_subscriptions.sidebar_hidden.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/routers/local_router.py

@@ -33,6 +33,7 @@ class CalendarUpdate(BaseModel):
     name: Optional[str] = None
     color: Optional[str] = None
     enabled: Optional[bool] = None
+    sidebar_hidden: Optional[bool] = None
     reminders_enabled: Optional[bool] = None
     caldav_published: Optional[bool] = None
     is_birthday: Optional[bool] = None
@@ -88,6 +89,7 @@ def _cal_dict(cal: models.LocalCalendar, *, owned: bool = True,
         # A recipient's own colour for a shared calendar wins over the owner's.
         "color": color_override or cal.color,
         "enabled": cal.enabled,
+        "sidebar_hidden": bool(cal.sidebar_hidden),
         "reminders_enabled": bool(cal.reminders_enabled),
         "caldav_published": bool(cal.caldav_published),
         "is_birthday": bool(cal.is_birthday),
@@ -265,6 +267,8 @@ def update_calendar(
         cal.color = data.color
     if data.enabled is not None:
         cal.enabled = data.enabled
+    if data.sidebar_hidden is not None:
+        cal.sidebar_hidden = data.sidebar_hidden
     if data.reminders_enabled is not None:
         cal.reminders_enabled = data.reminders_enabled
     if data.is_birthday is not None: